The present invention is concerned with an image forming apparatus of the type wherein toner images are transferred to copy paper electrostatically.
Such apparatus usually comprise a rotatable photosensitive member and are adapted to form images on paper by the steps of charging the surface of the photosensitive member, exposing the charged surface to an optical image to form an electrostatic latent image on the surface, developing the latent image with a toner, transferring the toner image to copy paper, removing the remaining surface charge from the photosensitive member and cleaning the member.
Of these steps, the charging step and the transferring step, wherein static electricity is generated, are performed by a charging unit and a transfer unit each comprising a wire electrode. When a high voltage is applied to these units from a high-voltage source connected thereto, the wire electrodes generate static electricity through corona discharge.
However, the wire electrode is likely to deteriorate owing to soiling with toner or corrosion, or to break owing to a paper jam. If the wire electrode breaks, causing short-circuiting, a great current flows and is therefore very hazardous. To eliminate the hazard, the high-voltage source is usually provided with a short detecting circuit for bringing the voltage source out of operation upon detecting short-circuiting.
While many of the apparatus of the type mentioned are sequentially controlled by a microprocessor, the operation of the short detecting circuit is monitored by the microprocessor at all times. For example, the processor judges whether a particular short is due to a break of the wire electrode, whereupon a signal is produced to indicate whether the electrode has broken (Japanese Laid-Open Patent Application No. 54-21727).
Thus, the system including the high-voltage source (hereinafter referred to as a "high-voltage unit" since the system is generally in the form of a unit) must be provided with the short detecting circuit, and further with a "break" indicating signal output means for which a greatly different voltage value is used, when the apparatus is under the control of a microprocessor. The high-voltage unit itself therefore requires an increased cost.
The short detecting circuit detects a break of the wire electrode only when the broken electrode contacts a conductor to cause short-circuiting, and is unable to function otherwise, for example, in the event of a mere break (without the contact of the electrode with a conductor). In the latter case, the copy discharged from the apparatus usually indicates the trouble to the operator, whereas the apparatus is sometimes operated in the absence of the operator. In view of these situations, it is desired to provide a system which is basically adapted to detect the troubles in the overall high-voltage unit including the break of the wire electrode, etc.